Conventionally, CVD apparatuses such as MOCVD and MOVPE, MBE apparatus and the like for epitaxial growth have been used for the production of semiconductor single crystals such as Si, GaN, SiC and the like. For production of SiC, a sublimation method, an HTCVD method (high temperature CVD method) and the like, requiring a high temperature of not less than 1500° C., particularly not less than 1800° C., are often used. For production of these semiconductor single crystals, hydrogen, ammonia, hydrocarbon gas and the like are generally used as a carrier gas and a starting material gas.
Carbon materials undergo gasification reactions at a high temperature of not less than 800° C. due to ammonia and hydrogen gas and are converted to methane gas, thus resulting in the volume change and weight decrease. Volume change causes, for example, change in the resistance of heater, which in turn varies process temperature. Consequently, degradation of the quality of epitaxial growth layer is feared. In addition, it is feared that, due to the volume change, a contact surface of a susceptor retaining a crystal wafer with a wafer becomes rough, which in turn causes non-uniformity of the temperature distribution of wafer, thereby consequently causing a defective epitaxial growth layer. The reaction between a carbon material and a gas is further accelerated particularly at not less than 1000° C. In this case, the heater and susceptor are deteriorated extremely in a short time. To suppress methanation of carbon materials, composite materials obtained by coating carbon substrates with dense silicon carbide by the CVD method have been used as furnace inside materials such as susceptor, heater and the like. However, the gasification reaction of silicon carbideat begins at 1300° C., silicon carbide is gasified by hydrogen at a high temperature of not less than 1500° C. and corroded at a rate of 5-30 μm/h. As a result of the corrosion, the coating film of the susceptor contains cracks and delamination, which allow corrosion of inside carbon materials. At this point, gases such as N2, O2, CO2 and the like remaining in the carbon materials are released and feared to be incorporated in the crystals for semiconductor devices. Such gases can be the cause of defective doping of the semiconductor device to be finally obtained.
The wafer surface sometimes becomes flawed during raising the temperature of single crystal wafer for crystal growth or cooling the wafer to room temperature after crystal growth. Such flaw can be caused by etching of the wafer surface by a carrier gas and the like or sublimation and dissociation of the atom on the wafer surface. Such flaws are not preferable since they degrade the properties of the device and increase the contact resistance. Recently, the development of the surface flaws has been decreased by accelerating the temperature rise rate and temperature decrease rate during crystal growth, in other words, shortening the time for temperature rise and temperature decrease. In some cases, moreover, a wafer may be produced in a short time by rapid temperature rise and rapid temperature decrease to simply improve production efficiency. Rapid temperature rise and rapid temperature decrease in this way gives rise to a new problem of delamination and cracks produced in a susceptor which is a part of an apparatus for forming a single crystal. This is because a considerable thermal stress is generated in the susceptor.
In the case of epitaxial growth of GaN, for example, a sapphire substrate is heated to 1200° C., and then cooled to room temperature. At this time, the temperature of the susceptor is rapidly risen and rapidly lowered. It is feared that cracks may occur in the coating film of the material of the susceptor due to the repeated temperature rise and the temperature decrease. Hydrogen gas and ammonia gas as carrier gases and starting material gases may penetrate into the inside of the susceptor through the cracks. The graphite materials that form the substrate of the susceptor are gasified by these gases, and the aforementioned undesired results are feared to be produced.
Therefore, to enhance the corrosion resistance of the heater, susceptor and the like, coating of carbon materials with a tantalum carbide layer has been tried. According to the disclosure of JP-A-10-236892 and JP-A-10-245285, a carbon material coated with a film formed by deposition of tantalum carbide fine particles by the AIP method affords heaters and susceptors that can be used longer than conventional ones. Moreover, the CVD method enables formation of a coating film of tantalum carbide, which is dense and superior in corrosion resistance. Therefore, a long-life carbon material is expected to be provided, since a coating film made of TaC having high crystallinity can be easily obtained by the CVD method. However, a coating film having high crystallinity, which is obtained by the CVD method, has a columnar structure and low flexibility and easily produces cracks. When ammonia gas and hydrogen gas corrode the carbon substrate through cracks, the life of the carbon material becomes short.
Thus, an attempt has been made to reduce the crystallinity of tantalum carbide of the coating film obtained by the CVD method, thereby affording a near amorphous state of the coating film to suppress occurrence of cracks and delamination (JP-A-2004-84057). The resulting coating film made of tantalum carbide is superior in density and flexibility.